The present invention relates to a process for the treatment of a purge solution making use of a process for electro-extraction of a recoverable metal, such as zinc associated with a process for membrane electrolysis.
This process of electrolytic treatment, the aim of which is to produce a purge solution depleted in zinc and in sulfuric acid, will hereafter be called extraction by electro-electrodialysis.
The invention also relates to a process for mounting an ion exchange membrane.
The manufacture of zinc by a hydrometallurgical and electrolytic route comprises a final operation of processing by electrolysis of solutions obtained by sulfuric leaching of roasted sulfide ores. Some of the impurities in the ores pass into solution during the leaching and more or less completely escape the purification process which precedes the electrolysis. Consequently, the impurities which are not deposited at the electrodes tend to concentrate in the electrolyte. When their concentration becomes too high, the solubility of zinc sulfate decreases and they tend to disturb the course of the electrolytic process. It is therefore necessary to carry out a "purge" of a fraction of the electrolyte. These purges cause major losses of zinc and sulfuric acid and in addition they have the disadvantage of being highly polluting.
The invention relates to a special treatment of a purge solution which has been withdrawn from an individual stage of the extraction process.
Understanding of the essential characteristics of the invention requires first of all knowledge of the usual processes employed in the technology. FIG. 1 is an outline diagram illustrating an example taken from the conventional processes for extraction of zinc by electrolysis. Reference 10 indicates the roasted sulfide ores forming the essential raw material. These ores undergo a leaching 12 intended to solubilize the zinc to the maximum and to retard the dissolution of the impurities as much as is possible. In general, the leaching comprises three operations: a "neutral" leaching operation 12a, an "acid" leaching operation 12b and an iron precipitation operation 12c. In practice, the solution obtained after acid leaching and precipitation or iron is subjected to neutral leaching. The raw leachate solution formed by this neutral leaching, indicated by the reference 14, arrives at the purification operations marked by the general reference 16. These operations are intended for the practically complete precipitation of the impurities which can be dangerous for the electrolysis, in particular, of copper, cadmium, nickel, cobalt, and the like. The solution 18 which is formed is a purified solution rich in zinc sulfate. This solution is then subjected to electrolysis 20. For example, the solution undergoes several electrolyses in cascade as shown by the references 20a and 20b. The zinc is deposited on the cathodes and the depleted solution 22 which has undergone electrolysis contains a large quantity of sulfuric acid and it is reused for leaching the ores 10. It will therefore be noted that the processing is carried out in a closed loop, with the result that the impurities which do not disappear during the purification 16, during the electrolysis 20, or during the precipitation of the residues (12b, 12c) accumulate and can attain very high values.
These leachate residues contain the iron entering the ore in various forms depending on the extraction process employed. The iron can be insolubilized in the form of goethite, hematite or jarosite. In the case of jarosite, we find, in association with the iron, alkaline elements (Na.sup.+, K.sup.+ and NH.sub.4.sup.+), sulfate ions (SO.sub.4.sup.2-) and water. This method of removal can be more advantageous for making use of the extraction by electro-electrodialysis.
The rates of purging allowing the level of residual impurities to be maintained below permissible limiting concentrations will thus be determined. The main impurity, and the one which determines the rate of purging of the plant is generally magnesium, since the great majority of zinc ores contain magnesium. The use of electrolytic extraction of zinc begins to pose problems when the concentration of magnesium exceeds 15 to 20 grams per liter. The problem posed by the magnesium grows when the concentrates employed as raw materials are of the dolomite type.
Another impurity whose accumulation is liable to pose some problems is manganese. The presence of this element is necessary, but it also must not exceed a specified concentration. The halogens, particularly fluorine and chlorine, can also accumulate in the electrolyte and become a nuisance for the electrolysis of zinc sulfate. However, since it is magnesium which most frequently determines the rate of purging of a plant for the extraction of zinc by electrolysis, the invention is described with reference to the separation of magnesium. However, those skilled in the art will easily be able to note that it also applies to the other impurities which can accumulate.
The purge solution may be withdrawn at one or more different locations in the operation of the process. For example, the purge 24 can correspond to a withdrawal of the raw leachate solution which is rich in zinc sulfate. As shown by reference 26, the purge can refer to a part of the purified solution rich in zinc sulfate. The purge can also be carried out during electrolysis, between several treatment stages as shown by reference 28. However, most frequently, the purge applies to the solution depleted in zinc sulfate which has just undergone electrolysis, as shown by reference 30. It may be considered that this purge of the depleted solution is the most judicious because it is the solution which contains the least zinc which constitutes the useful product. Nevertheless, this solution is highly acidic and requires the use of a large quantity of neutralizing agents.
The processes which are generally employed in the technology for the treatment of purges are, on the one hand, a neutralization-precipitation process and, on the other hand, a process of preliminary leaching of the concentrates. The neutralization-precipitation may sometimes be preceded by an electrolytic extraction of the solution.
In some rare cases, the purge solution 30 can be marketed directly. For example, the neutral purified solution of zinc sulfate can sometimes be employed directly for the production of zinc salts or of lithopone. Similarly, products obtained by straightforward evaporation of the solutions can sometimes be sold. However, these are relatively rare cases, taking account of the small market for such products.
All these solutions, however, are tied to an external market or to a particular environment of the zinc electrolysis plants and are subject to changes which can, in certain cases, affect the operation of the main process. One of the objectives of the present invention is to provide a purge treatment process which is integrated into the main scheme.
Unconventional treatments of the treatment of these purge solutions have also been tried. Reference can be made for example to the direct extraction of zinc by fixing on ion exchange resins or liquid-liquid extraction. Reference can also be made to the reversible fixing of sulfuric acid on ion exchange resins. However, the latter processes have not yet met with any real industrial success.
Attempts have also been made to treat the depleted solutions by dialysis and by electrodialysis. The dialysis process permits the formation of a moderately dilute acid which can be recycled, and of a low-acidity solution containing all the metallic cations. Operations of neutralization, solvent extraction, and the like, then permit a zinc extraction. The electrodialysis treatment permits the formation of recyclable sulfuric acid and the rejection of a magnesium-containing purge with low acidity which can be neutralized. These processes have not however met with commercial success.